1. Field of the Invention
The present invention relates to a heat machine. Such a heat machine may be used as a heat engine for converting heat to useful work or as a heat pump for moving heat from one place to another. For instance, such a heat pump may be used as a cooling or refrigeration apparatus based on the Stirling cycle.
2. Description of the Related Art
A known heat pump or "cryoengine" of the split Stirling cycle type is used for cooling an infra-red sensor to low temperatures, for instance of the order of several tens of degrees Kelvin. A "cold finger" is connected to a remote source of gas, such as helium, whose pressure is cyclically varied. The cold finger contains a displacer and heat regenerator which cool the tip of the finger, which tip is in contact with the sensing element of the infra red sensor.
In order to improve the efficiency of such heat pumps, it is known to provide a two-stage pump having two displacers and regenerators. For instance, U.S. Pat. No. 4090859 discloses a cold finger in which two displacers, each containing a heat regenerator, are arranged for free movement "in-line". However, such an arrangement creates problems in balancing the reciprocating movement of the two displacers, and requires a relatively complex construction.
U.S. Pat. No. 4425764 discloses an arrangement in which a first displacer is provided with an external heat regenerator and contains a second much smaller displacer which contains its own heat regenerator. This arrangement allows displacers of relatively low weight to be provided so as to reduce the reciprocating masses. However, two separate working gas feeds are required to the two displacers as the pressure variations required by the displacers are not in phase.
It is known to provide a two stage heat pump in which two displacers are fixed together but axially displaced from each other. However, this causes balancing problems because, in practice, the center of gravity of such an elongate displacer is located at a substantial distance from a spring suspension system for the displacer. Although counter-weights may be added to the displacer in order to move the center of gravity closer to the spring suspension, this increases the mass of the displacer, whose reciprocating motion therefore increases vibration of the cold finger. It is therefore more difficult to provide a stable mount for an infra-red sensor which is cooled by such a cold finger.